Improve LED brightness in battery-powered devices

Screen brightness is particularly important in LED-backlit LCD displays for portable devices. Brightness is controlled via an LED panel mounted on the rear of the LCD, which boosts the input voltage connected to the panel. For the LED panel's operation, a boost converter converts the input voltage to dozens of volts. To prevent brightness flickering and control the stability of the LED output voltage, stable current resistors with low thermal EMF and low absolute TCR are required. 

The LED1625 features a 1W power rating and 0.3-10Ω resistance range, while the LED221 features a power rating of up to 8W and a resistance range of 0.5-500Ω. Any value within the devices' resistance ranges, to six digits, is available at no additional cost. At rated power, the resistors offer PCR of 5ppm.

The devices, which feature a non-inductive, non-capacitive design, offer a rise time of 1ns with effectively no ringing. The LED1625 and LED221 also provide voltage coefficient of less than 0.1ppm/V, current noise of 0.010μVRMS/V and thermal EMF of less than 0.05μV/°C. Withstanding ESD spikes to at least 25kV, the resistors are available with lead (Pb)-free and tin/lead alloy terminal finishes. The LED221 is also available with a solderable heatsink tab.

"Battery life is critical in portable battery-powered applications, so for an LED driver to be useful, it must be efficient," said Hero Faierstain, Senior Manager, Application Engineering, VFR. "Power efficiency is largely determined by the precision of the four-terminal current sensing resistor and brightness control. The amount of input power required to generate the desired LED brightness can be determined by dividing the power in the LEDs by the input power. Defining the efficiency in this way means that the power dissipated in the current sensing resistor contributes to the power lost in the power supply. Therefore, it is very important to use high-precision current sense resistors, such as the LED1625 and LED221."

"Beginning in 2015, Farnell element14 will be able to ship the LED Series resistors with no minimum quantities," said Jacky Acoca, Sales and Marketing Director, VFR. "Through collaboration with VPG, Farnell element14 is helping to set a new standard by shipping the exact number of resistors needed of the precise resistance value and tolerances required."

The LED1625 and LED221 are available now in both sample and production quantities.